Spark gap



F. L. JONES Dec. 21,1948.'

SPARK GAP Filed latch 27, 1945 I y: ventor Attorney Patented Dec. 21,1948 SPARK GAP Frank Llewellyn Jones, Millbank, London, England,assigner to Minister of Supply in His Majestys Government of the UnitedKingdom of Great Britain and Northern Ireland, London,

England Application March 27, 1945, serial No. 585,165 In Great BritainFebruary 17, 1941 Section 1, Public Law 690, August 8, 1946 Patentexpires November 4,1962

2 Claims.

This invention relates to spark gaps of the kind used in high frequencyignition systems of internal combustion engines and in certain radiocircuits using rapid spark discharges.

An object of the invention is to provide a spark gap capable ofoperating at a high rate of sparking for many prolonged periods withoutsubstantial change in its electrical characteristics despite changes inexternal conditions such as barometric pressure, temperature, humidityor incident illumination. It is also an object of the invention that thegap should attain these characteristics within a minimum time of initialforming Another object of the invention is to provide a spark gap whichis almost a perfect insulator when the applied E. M. F. is less than acritical value but which rapidly breaks down and becomes a goodconductor when the E. M. F. rises above the critical value, thebreakdown potential of successive sparks lying within close limits, i.e. having an impulse ratio of nearby unity. Also, the gap must encouragethe passage of a spark, as distinct from an arc which might persist foran interval of time sufficient to prevent the gap from reverting to afully non-conducting condition preparatory to the application of thenext pulse of E. M. F.

A specific object of the invention is to provide a spark gap suitablefor use in a high frequency ignition system where a condenser, connectedinseries with the spark gap and with the primary coil of a highfrequency transformer, is charged from an impulsive source of E. M. F.,such as a magneto or ignition coil, to a potential diiference in excessof 1000 volts and preferably less than 5000 volts, and then dischargedacross the gap. For such a system to operate successfully the breakdownof the gap should occur within a very short interval of time, of theorder of -5 sec., after the applied E. M. F. has attained the criticalvalue below which the gap is an insulator, and there should be a maximumdifference of about 20% between the breakdown potentials of successivesparks.

A further specific object of the invention is to provide a spark gapwhich is particularly suitable for use in the high frequency ignitioncircuit of an aircraft in which case reliability becomes paramount andsmall dmensions are an advantage.

To attain these objects a spark gap constructed according to thisinvention is enclosed Within a gas-filled and gas-tight envelope inwhich the electrodes are sealed and rigidly supported. The

2 electrodes are shaped and disposed so that a practically uniformelectric eld is maintained throughout a substantial fraction of thespark gap volume, and are made from materials with thermionic workfunctions greater than 4.0 electron volts and with low erosionproperties under the action of the spark discharge. Also the prodf uctof the pressure of the gas filling measured at 15 C. and the distanceapart of the electrodes is greater than 5 times the value of thecorresponding product at the minimum sparking potential of the gas. Whenthe electrodes consist of two parallel opposed discs the ratio of thediameter of the discs to their distance apart is made not less than 10and is preferably greater and the distance apart of the electrodes ismade not less than approximately 0.2 mm. A hard glass which remainsrigid at temperatures up to about 450 C., or quartz, are suitablematerials for making the envelope. The gas or gases chosen to ll theenvelope are such that any chemical changes produced in the gas itself,or any chemical reaction between the gas and the materials inside theenvelope are negligible during the actual operation ofthe device.Suitable gases for lling the envelope are, for example, the monatomicgases, or hydrogen, or nitrogen, and nitrogen is especiallysuitable'when the improved device is used on an aircraft, while hydrogenis especially suitable when a very highrate of sparking is required. Thegas density is determinedby the purpose for which the enclosed spark gapis required, but it should not be less than that corresponding to 20 cm.of mercury at 15 C. and for very short spark breakdown times higher gasdensities are necessary. The most suitable gas density when the enclosedspark gap is used in a high frequency ignition system is thatcorresponding approximately to atmospheric pressure, and a pressureabout 5% or 10% less than atmospheric is especially suitable, as then inthe process of manufacture the process of sealing of the envelope isthereby simplified, and further, high internal pressures, which tend tobreak the envelope, are less likely to occur during the operation of thedevice. In the case of hydrogen or nitrogen the value oftheaforementioned product which corresponds to the minimum sparkingpotential is about 1, when the gas pressureis measured in cm. of mercuryat 15 C. and the distance apart of the electrodes is measured in mm., sothat when hydrogen or nitrogen is used in the improved spark gap thevalue of the product should not be less than about 5.

The seals through which the connections to 3 the electrodes are broughtout through the envelope, which may also be the means by which theelectrodes are rigidly supported; should be mechanically strong andcapable of working at temperatures up to about 400 C. A suitableform ofseal is a tungsten-hard glass seal, for example,

Two practical embodiments of the invention are illustrated in theaccompanying drawingin which- Figure l is a longitudinal section of a4form of the invention in which the electrodes consist of parallelopposed discs and Figure 2 is asimilar view of a modified form of theinvention inwhich the electrodes consist of coaxial cylinders. Workableoverall dimensions ofV the. device in each case may be about 31/2 in.long by 1 in. diameter. v

In Figure 1 the electrodes I and 2 consist of discs with rounded edgesmade of a metal having a thermionic work function greater than 410electron volts, such as tungsten. The diameter of fthe hat parts' of thediscs is indicated by the letter D and the overall diameter by DI. Thedistance between theelectrodes is indicated by the small letter d.'

'Ihefdiscs` are-welded to tungsten rods 3 and 4 respectively, the crosssectional area of which determines the rate of `cooling of theelectrodes, andtherodsextend through a hard glass envelope 5. The rodsare sealed directly into the envelope the-walls of which are suitablythickened at B and Iso as to increase the lengths or the seal-and giveenhanced mechanical support.

After filling withI gas the envelope is perma-` nently sealed atL 8:

In the modification illustrated in Fig. 2 the electrodes consistof-'coaxial cylinders 9 and I0 made of a-metal having athermionic workfunction greaterrthan 4.0 electron volts and' spaced-y apartby adistance d. The cylinders are supported on metal rods 3 and 4respectively, which may be similar to the rods 3 and diin Figure 1, andare enclosedin a hard glass envelope'into whichthe rods are sealed `atthe thickened parts` G" and 1.

The exactshape of the electrodes-is not important provided'that they areshaped and so disposed that a practically `uniform electric field ismaintained over ay substantial portion of the spark gap volumel When,however, the enclosed sparkgap is to bel operatedfrom a source ofalternating polarity, such as a-magneto fork instance, then it isessential that the electric'i'leld should be the same at each electrodesurface and therefore the electrode arrangement shown in Fig. 1 is to bepreferred in such circumstances. If the electric field werenot uniformbut were concentrated more strongly at one electrode than at the Other,then the sparking potential would not be the same wheneither electrodeis used'as the cathode,` so that when operated from a magneto thebreakdown potentials for successive sparks would in general bedifferent, and this would be a disadvantage whenthe-enclosed spark` gapis` used in a-high frequency ignition circuit. Again, under theseconditions itis preferable that both electrodes should be made-of thesame material so that they have the same thermionic work functions. Thereason for this is that'durng `the operation of the spark gap at a highrate of sparking, say in excess-of about'lOO sparks per second, alargerise of temperatureofthe electrodes can occur unlesstheheat'is'conducted away. Under these conditions considerableth'ermionic emission occurs from the opposed surface of the electrodes.This thermionic emission is of great importance in the operation of thespark gap. Firstly, it can create the supply of electrons in the gapwhich is necessary for its rapid breakdown when the applied potentialexceeds a critical value which is approximately equal to the sparkingpotential of the gap under a steady applied potential. If the breakdownpotential of the spark gap is to be the same when either electrode isused as the cathode, thenthe thermionic emission and the rate ofconduction oi heat away must be the same at each electrode. Thisnecessitates a symmetrical arrangement of the electrodes. Secondly, ifthe supply of electrons is too great and the gap is operating at a veryhigh rate of sparking, then in certain gases the electrical dischargetends to become a practically continuous arc instead of a succession oidistinct sparks. This is a great disadvantage as it renders a highfrequency ignition system inoperative, and it also generates aconsiderable quantity of heat, which may damage the seals and alsoproduce excessive erosion of the electrodes. Excessive therrnionicemission into the gap is reduced by employing electrodes of highthermionic Work function not less than 4.0 electron volts, and byensuring sufficient conduction of heat away from the electrodes. Adegree oi thermionic emission is, however, in some cases an advantage inorder to reduce the statistical time lag of sparking and it may benecessary to adjust the cooling of the electrodes to ensure that thetotal time lag of sparking is less than 10'4 second, and in general thelimit of 10-5 second is preferred. Excessive erosion is reduced byemploying metals of high boiling point for the electrodes. TheseConditions are satisfied by using, for example, metals like tungsten,molybdenum or platinum for the electrode material, and by using, forexample metals like 'tungsten for the supports for the electrodes.Tungsten is especially suitable for the electrodes and the supports,since this metal is a good conductor of heat and electricity, and it hasa high boiling point probably in excess of 4000" C. Also (Handbook or"Chemistry and Physics, Hodgman, Ohio. 1940, p. 1737), and Becker (Reviewof Modern Physics 7, 123J 1935) gives its work function as 4.52 electronVolts. Further, since its coefficient `of expansion is approximatelyequal to that of ywith advantage be made since a hard glass ismechanically strong and also can withstand temperatures up to about1:00o C. Another advantage of the use of tungsten for the electrodesupports is that the expansion of the supports is then approximately thesame as that of the glass envelope, so that the gap distance is notgreatly altered by any change in operating temperature of the enclosedspark gap. The electrical connections from outside the envelope to theelectrodes inside must be of low resistance, since large currents of theorder of amperes may be passed lwhen the spark gap breaks down. Also,the connections to the electrodes must be ci adequate size in order toensure suilcient cooling of the electrodes.

A further advantage of the use of materials of high work functionsinside the envelope is that the enclosed spark gap has no photo-electricproperties due to ordinary daylight, so that its characteristics are thesame Whether it is operated in the dark or exposed to ordinary daylight.

When the enclosed `spark gap is used in a circuit in which the polarityof .each electrode does not change, a perfectly symmetrical dispositionof the electrodes is not then essential, and they may consist of opposedsurfaces of different curvature. For instance, as shown in Figure 2, theelectrodes may then consist of coaxial cylinders when the radii ofcurvature of the opposed surfaces are great compared with their distanced apart. In all cases, however, sharp points or edges should be avoided,and the electrodes and their connections must be large enough to ensureadequate cooling of the electrodes and a lowresistance electricalconnection to them. l

A practically uniform electric eld in the spark gap is ensured by theelectrode arrangements shown in figure 1, i. e. by using as electrodestwo parallel opposed discs of large diameter compared .with theirdistance, apart. The effect of the value ofthe ratio diameter/distanceapart on the uniformity of the electri-c field in the gap has beenstudied by McCallum and Klatzow (Philosophical Magazine 1'7, 291, 1934)who showed that the sparking potential of a gas between parallel discsis only accurately a function of the product of the gas pressure,measured at 15 C., and the distance apart when the ratio exceeds acertain value dependent on the nature of the gas. In neon, for example,the ratio must exceed 13, while inargon it must exceed 40, but in heliumit is much longer. Hence, when the electrodes of the improved spark gap`are in the forrn of opposed, parallel discs as shown in Figure 1, theratio of the diameter D of the flat parallel surfaces to their distanceapart d will depend on the gas used to lill the envelope, the ratiobeing higher when the noble gases neon or argon, for example, are used,than when the diatomic gases such as hydrogen or nitrogen are used. Inall cases the ratio should exceed a value of about and preferably bemuch greater.

The edges of the electrodes should be rounded off with a gentlecurvature to avoid sharp points or edges, which should also be avoidedon the supports or connections of the electrodes inside the tube andwhen the edges of the discs are rounded off, the ratio of the overalldiameter DI of the electrodes to the distance apart d should then exceedabout 15.

The flat parts of the electrode surfaces should be spaced from theenvelope at a distance not less than 5 times the distance apart of theelectrodes.

The distance apart of the electrodes should not be less thanapproximately 0.2 mm. when the enclosed spark gap is to be used in highfrequency ignition systems, and the gap may be as great as 1.5 mm. whenthe enclosed spark gap is to be used for other purposes, such as inradio circuits, when a high sparking rate and a breakdown potential ashigh as 15,000 volts may be required.

To ensure as rapid a breakdown as possible after the applied potentialhas exceeded a certain critical value it is advisable to have the gaspressure as high as possible provided this does not require a gapdistance smaller than approximately 0.2 mm.

As hereinbefore stated the product of the dis-4 tance apart of theelectrodes measured in mm. and the gas pressure measured in cm. ofmercury at C. should exceed about 5 times the value of the correspondingproduct at the minimum sparking potential of the gas. For example, thevalue of this product at the minimum sparking potential for hydrogenwith various cathode materials has been found by Llewellyn-Jones andHenderson (Philosophical Magazine 28, DI).v 185, L.

192 and 328, 1939) to be about 1.2 and the value of the productcorresponding to the minimum sparking potential of nitrogen has beenfound by Strutt (Philosophical Transactions of the Royal Society A, 193,377, 1900) to be approximately equal to 0.8, so that when hydrogen isused as the gas filling the value of the product should exceed about 6,and when nitrogen is used the value of the produ-ct should exceed about4. Consequently when a gap distance of 0.2 mm. is used, vthe pressure ofnitrogen at 15J C. in the envelope should exceed 20 cms. of mercury.

The relationships between the gas pressure, p, measured in cm. ofmercury at 15 C., and the gap distance d, measured in mm. and the workfunction w measured in electron Volts may be expressed by the followingformulae:

(p) X (d) X (w) C parallel flat discs with a diameter of the flat partof the surface of D cm. then (p) X (d) X (w) v (D/d)=(p) X (w) X (D) (K)where K depends on the gas.

For example, with hydrogen,

(p) (w) (D) 240 and for nitrogen,

(il) (w) (D) During the initial operation of an enclosed spark gapconstructed according to the invention a fine deposit is sometimes, butnot always, automatically produced onor near the electrodes and walls,probably due to the evolution of absorbed gases, and their reaction withthe material of the electrodes when these have not been de-gassed. Asthe process continues for some time, which may be long, the electricalcharacteristics of the enclosed spark gap improve, and when the processis completed the characteristics remain steady throughout the life ofthe spark gap. When the electrodes are made from tungsten this depositcontains oxides of tungsten produced by the sparks in the presence oftraces of oxygen and water vapour which are generally present in theenclosed spark gap when it has not been degassed. These traces may befound as impurities, for inst-ance, in the preferred gas filler, or theymay have been absorbed by the electrodes and envelope and evolved duringthe operation of the gap, or they maybe liberated from the electrodeswhich may have become oxidised during the sealing-in process in theconstruction of the tube, or they may be liberated by the brazing orjoint fixing the electrodes to the supports. These final traces of watervapour and of oxygen and other occluded gases are not easy to remove,and any such removing process would require considerable time. However,in some casesy it is of advantage not to attempt to remove them, but toutilise them in the operation of the gap. In the first place, thepresence of water vapour readily assists the formation of oxides, whichin some cases may distil off from the electrodes due to the passage ofsparks. Secondly, the presence of water vapourassists arc suppressionowing to the electron amnity of water molecules. Naturally, the depositsare quickly produced if oxygen, or air are used in the gas filler andtraces of water vapour are present.

The deposits on the electrodes can have various eiects on the operationof the tube. For instance, they can raise the thermionic work functionof the surface and therefore have the advantage of delaying thetransition to a thermionic arc immediately after breakdown withconsequent reduction in the rate of liberation of heat from the gap.They also cause successive sparks to pass to dilerent points on theelectrodes and so avoid excessive heating of one particular part. Infact an enclosed spark has been operated at 400 sparks per second, andthe resulting rise of temperature was found to be about 30 C. to 40 C.Again, the deposits have the Well-known effect of tending to produceconstancy in the break-down potential of successive sparks and reducingthe statistical time lag, (See for instance H. Paetov, Zeitschrift frPhysik, volume III, p. 770, 1939, also M. J. Druyvesteyn and F. M.Penning, Review of Modern Physics, volume 12, No. 2, p. 117, 1940.)

Further, as the oxides become deposited on the nearby inner walls of theenvelope the deposits there can also exert influence on the operation ofthe spark gap. In the first place there is the advantage that ionisingradiations may be produced which tend to reduce the statistical time lagof sparking. (See H. Raether, Zeitschrift fr Physik, volume 110, p. 611,1938.) However, in the second place, if the deposits are allowed to betoo near the region of uniform eld in the gap the statistical lag can beincreased, consequently the impulse ratio is raised. To reduce thiseffect to a minimum the distance of the walls of the envelope from theregion of uniform field should, as stated hereinbefore, be five timesthe length of the discharge gap. Deposits on the walls of the envelopenear the gap may help in the dissipation of charges from that region,collected there by diffusion from the ionising path o1' the electricaldischarge. Recombination of ions and electrons at the walls can beregulated by the proximity of the walls to the discharge path. (See F.Llewellyn Jones, Philosophical Magazine, volume 15, p. 958, 1933) andthis diffusion and recombination can play an important `part in therapid de-ionising of the gap after each spark has passed. An enclosedspark gap in accordance with the invention using nitrogen and tungstenelectrodes has been constructed for which the variation of breakdownpotential for successive spark was about 1% to 2%. Once the suitabledeposits are formed, their influence remains unaltered during the lifeof the enclosed spark gap with a suitable gas ller. With nitrogen ofcommercial purity, for instance, the effect of any chemical action, ifany, between the gas and the contents of the envelope is negligible, andno such effect attributable to that cause was observed during theoperation of an improved tungsten electrode spark gap for more than 300hours.

The evolution of the usual absorbed gases by the electrodes andenvelope, or the presence of traces of oxygen and water vapour in thegas ller renders the production of the deposits of oxides automaticduring the initial stages of operation of the enclosed spark gap, butthe process may take considerable time, and this would be a disadvantagein large scale production. This time can be considerably reduced, oreven eliminated, by first depositing a fine layer of the appropriateoxides over the electrodes during the assembly of the enclosed sparkgap, and before nally sealing off the envelope. During the passage ofthe first few sparks across the gap with tungsten electrodes, forinstance, some of these deposits are vaporised and condensed on the envelope.

The effect on the sparking potential of the presence of mercurydeposited on the cathode surface has been investigated by LlewellynJones and Galloway (Proceeding of the Physical Society, 50, 207,1938),and the effect of the presence of sodium deposited on the cathodehas been investigated by Ehrenkranz (Physical Review 55, 219, 1939), andin both cases the sparking potential with a cold cathode wasconsiderably reduced due to the lowering of the work function of theelectrode. Even when the electrodes are hot, as would be the case whenthe enclosed spark gap is operating at a high rate of sparking, thepresence of the metallic vapours in high concentration tends tointroduce variations in the thermionic work functions of the electrodesand also in the electrical properties of the gas, such as the ionisationcoefficients for example, and consequently in the value of the breakdownpotential.

A further disadvantage of the use of any metallic vapours in theenclosed spark gap is due to the variation of density of the metallicvapours caused by the operation of the enclosed spark at widely diierenttemperatures. For instance, if the enclosed spark gap were fitted to anaircraft the temperature of the atmosphere surrounding the envelopewould undergo large variations due to the operation of the aircraft atdifferent altitudes and under different atmospheric conditions.

The nature and pressure of the gas filling is determined by the purposefor which the enclosed spark gap is to be used. When the en closed sparkgap is toibe used in a high irequency ignition system and the breakdownpotential of the spark gap lies within the approxi mate limits 1000volts and 500() volts, the gas used must be such that the gap distanceunder these conditions must exceed 0.2 mm. when the gas pressuremeasured at 15o C. is greater than 20 cm. of mercury and preferably lessthan about 76 cm. of mercury. Also, the gas must allow separate sparksto pass when the rate of sparking does not exceed 400 to 800 sparks persecond depending on the type of engine. Consequently, it follows thatthe deionsation of the gap must be so complete after the initial appliedpulse of electromotive force which produces a spark has passed, that thedischarge does not persist until the next spark produced by the nextpulse of electromotive force from the magneto, or ignition coil or othermeans which produces the potential necessary to break down the sparkgap. If the gas atoms or molecules have metastable states, and if thegas contains traces of impurities and if the potential energy of themetastable levels is lgreater `than the impurity, the ionisation maypersist in the gap after the applied electromotive force has beenreduced to zero, due to the process of ionisation by collisions of thesecond kind. A method of reducing the persistence of ionisation in gaseswith high metastable energies is the well known one of destroying themetastable atoms, where possible, by lprocesses which do not lead toionisation, and such processes can occur, for example, when thepotential energyof the metastable atom is less than the ionisationpotential of an impurity molecule with which it collides. Further, `therate of de-ionisation ofthe gap after a spark has passed depends on thecoeflicients of recombination and of diiusion of the electrons and thepositive and negative ions in the gas used. These coeilicients,generally, are higher in a` light gas than in a heavier gas,consequently shorter deionisation times are to be expected in hydrogenthan in nitrogen, so that when very high rates of sparking are requiredit would be of advantage to use hydrogen as the gas lling. However, whenthe enclosed spark gap is used in a high frequency ignition system on anaircraft, the use of nitrogen as a filling'. is preferable. This is dueto the fact that the sparking potential of nitrogen is not verydifferent -fromthat of air under similar conditions. Consequently, ifnitrogen at a pressure of about '70 cm. of mercury were used in theenclosed spark gap, the breakdownv potential of the gap would not rbegreatly altered at low altitudes if by any chance the envelope. weredamaged so as to admit air which would 'contaminate the nitrogen andalso would equalise the internal gas pressure with that of thesurrounding atmosphere, provided the position of the electrodes wasunaltered. Such a form of damage would be a crack in the envelope or afailure of a metal-glass seal, thoughsuch damage is not likely to occurduring the normal operation of the tube. Consequentlmthe sparking plugsin the engine would continue to ire in spite of the damaged envelope. Onthe other hand, if the damage to the envelope occurred at yhighaltitudes when the atmospheric pressure was greatly reduced, say toabout 0.1 of the pressure at sea level, the breakdown potential of theenclosed spark gap would tend to be so low that insufficientelectromotive force would be generated at the sparkingy plugs andinisring consequently occur. However, when the aircraft had descended tothe lower altitudes, regular rlng would again be produced in the enginewhen the former breakdown potential of the enclosed spark gap wasrestored by the higher atmospheric pressure at the lower altitude nearthe ground. In this way the chance of an aircraft crashing due to anengine mis-fire `produced by such damage to the envelope' of the ienclosed spark gap could be minimised.

Hence, when tted to an aircraft, the gas filling for the improvedenclosed spark gap should, as a safety measure, preferably be nitrogenat a pressure within the limits of 50 cm. to 100 em. of mercury at 15C., but a pressure of about '70 cm. of mercury is preferred. However,under conditions when the life of the enclosed spark gap under operatingconditions need not be so long as it would be if nitrogen were used asthe gas lling, then air could be used at pressures a little less thannormal atmospheric. This would be the case when the rate of sparking isvery low, such as when the enclosed spark gap is used as a lightningarrestor.

If the enclosed spark gap is used in a high frequency ignition system onan aircraft, for instance, it is advisable that the risk of a misre inthe spark plugs of the engine should be reduced as much as possible.Such a misflre would occur, for instance, if the breakdown potential ofthe enclosed spark gap were momentarily raised due to some cause so asto exceed the E. M. F. applied to the gap. Again, if the breakdownpotential of the enclosed spark gap varied from spark to spark in l0akf'sccessionf'ofbreakdown within limits, for example, of about 10%, dueto some cause, and if the applied E. M. F. fell within those limitsthere would then be risk of a misre. This risk can be reduced byemploying two or more improved spark gaps in parallel when the sparkgaps have electrical characteristics, including breakdown potential,impulse ratio,'and gap voltage-time curves, for instance which Vare asfar as possible the same for all the gaps. This `is due to the fact thatwhen the breakdown' potentials for successive sparks vary within limitsof say, about 10%, measurements of those breakdown potentials showthatthemost probable value of the breakdown potential is rather less thanthe arithmetic mean of the maximum and minimum values of the breakdownpotential.v In other words, if, for example, two practicallyidenticalenclosed spark gaps, in accordance with the invention, eachwith an impulse ratio of say 1.1, were 'used' in parallel the eiectiveimpulse ratio ofthepairwould be about 1.05, because the probability ofthe occurrence of an impulse ratio of 1.1 maybe neglected. Similarly, ifthe impulse ratio of each gap were about 1.05, then the impulse ratio ofthe pair used in parallel would be reduced to about`1.025 for the samereason. If, one gap failed to breakdown the other gap would beunaffected, andk would vstill allow the system dependent on the gap tooperate. f The success of such a combination of spark gaps is due to thefact that each gap has a (breakdown potentialrate of sparking)characteristic which rises slightly as the rate `of sparking increasesfrom aboutl 40 sparksf 'per 'secondtoabout 300 sparksper second, and isalso dueto the factthat an improved enclosed spark gap can beconstructed to have impulse ratios as low as about 1.02 and alsofas-hlghas about'1.2; When -using gaps in parallel it is not advisable to usegapst which have impulse ratios very near unity, andigood results havebeen obtained when using .two gaps each'with an impulse ratio as low asabout 1.05.

The gaps used in any such combination replace the' use lof a' .singleenclosed spark gap and the combination may consist of two or morecomplete and separate gaps, oralternatively they Ymay consist of two ormore pairs of electrodes in one envelopebutv the use of distinctandseparate enclosed gaps is preferred, since then one: gap can bedestroyed vor removed without upsetting the operationofthe,systemyprovided that the electrodes of the gap holden. are notshort circuited. Another advantage of the use of such a combination isthat a pair of gaps, for instance, can operate at a rate of sparkingtwice as great as that at which a single gap can operate satisfactorily.This is due to the fact that if one gap of such a pair commences tooperate at a very great number of sparks per second then its breakdownpotential tends to rise slightly. This will -then increase theprobability of the other gap in the pair breaking down whichconsequently will reduce the sparking rate of the first gap. In that waythere is a tendency to equalise the sparking rate for the two gaps in apair.

A further advantage of the use of a combination of gaps in the mannerindicated above is that, for a given rate of sparking required by thesystem utilising the gaps, the life of the combination of enclosed sparkgaps is greater than that of a single gap. Again, another advantage ofthe use of a combination of enclosed spark gaps is that overvoltagesconsiderably in exces of the breakdown potential of the gaps may beapplied to the combination with safety under circumstances 1.1 whichwould `tend to zproduce considerable Aheat and consequentdamageto a gap.acting alone.

If an enclosed spark gap constructedaszhereinbefore described hadlanenvelope `with overall dmensions of about -3" x t" x $5", lthen'theuse of a combination of four such gaps wouldoccupy arectangularvolumenotgreatly exceeding about 3" `x 1 x 1". Such acombination would have a fourfold increase in safety factor, aneffective breakdown voltage -variation of about one quarter f that ofthe single gap, and also .the ability to handle a spark .frequency ofthe ordervof `1600 sparks per second. Alternatively, an extremely long`life is to be expected :from thecombination at lower spark frequencies.

The use of sparkgaps in parallel has another advantage in that itintroduces the possibility of obtaining an effective spark gapwhich,-when used in `an ignition system on laircraftfor example, isprotected against a failure of any of=^the separate gaps to breakdown,and also against a ycrack in the envelope of any gap of the combination,provided `one enclosed sparkfgap ,is still intact. A cracked `envelopecan `equalise the pressure .inside the gap with that of the atmosphere.It :is only necessaryto use anenclosed sparkf gap in which the nature ofthe gas filler, anci'its density, and the gap distance are :so arranged`zthat, `when filled `Withatmospheric air at thedensity appropriate tothe highest operational `altitude considered, thebreakdown potential is:either equal to or higher than,zthat of theother gaps in thecombination, which `gaps then control'the sparking at all altitudes.Consequently, misiiring due either to a'cracked envelope or:failuretobreakdown in any1of the constituent gaps .isfthereby avoided.

When nttedto an aircraft Athe overall .dimensions of the enclosed sparkgapmust be such that the minimum distance `between the `exposed metalconnections *at theiseals, orany'other conductors connected to them,must ybe Asuch .that spark over between them does 'notioccur outside theenvelope whenthe enclosed spark gapis used at the highestoperationalraltitude, `or at a reduced atmospheric pressure surroundingthe envelope and connections.

The improved enclosed spark gap constructed as described above may bedesignedto operate at breakdown'voltagesln excess 'of 5000 voltsbyadjusting `thegap distance and/or the `density of the gas 1111er to havetheappropriate value. In

.order to produce the higher `breakdown potentials an increase inthe'gas density is, vin many cases, preferred, lwhen practicable, to anincrease in the gap distance, as this increases the value ofthefelectric intensity at the electrode surfaces, and it also helps tomaintain the dimensions of the electrodes reasonably small.

4I claim:

1. A spark `gap device comprising electrodes, arranged in parallelspaced relationshipsuch that a substantially uniform electric field iscreatable between them, made of materials having thermionic Work`functions greater than 4.0 electron volts, at least `one electrodesurface having deposited thereon an `oxide of the material of theelectrodeto enable the gapto operate at a high sparking rate, theelectrodes being rigidly mounted and spaced apart a distance greaterthan approximately 0.2 mm. within a sealed envelope filled with nitrogenat a pressure of be tween 50 and 100 cm. mercury at 15 C.

2. A sparkr gap device comprising two electrodes consisting of coaxial,telescopically arranged cylinders made of materials having thermionlcwork functions greater than `1.0 electron volts, at least one electrodesurface having deposited thereonan oxide of the material `of theelectrode to enablevthe gap `to operate at a high sparking rate, `theelectrodes being vrigidly mounted `and spaced apart a distance Adwithina sealed envelope filled with gas at a pressure -p such that theproductpd is greater than flve times the product p d which corresponds to theminimum sparking potential of the gas to prevent deleterious arcformation.

FRANK LLEWELLYN JONES.

REFERENCES CITED The following references are of record in the le `ofthis patent:

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